Method of fabricating an interconnection cable

ABSTRACT

A flexible connector cable for providing high density and reliable electrical interconnections between printed circuit boards or any other surfaces having conductive paths that need connection to conductive paths on adjacent surfaces. The connector cable comprises a flat flexible laminar structure including an electrically-insulative layer and an electrically-conductive layer. The insulative layer is typically formed on a bonded plastic such as Polyimide and the conductive layer is typically formed of copper. Openings are formed in the insulative layer to expose the conductive layer and raised contacts or buttons are deposited on the conductive layer on both surfaces of the cable. The raised contacts are formed of ductile conductive material which exhibits plastic deformation under pressure to form good electrical connections.

This is a division, of application Ser. No. 841,917, filed Oct. 13, 1977and now U.S. Pat. No. 4,184,729 issued Jan. 22, 1980.

FIELD OF THE INVENTION

This invention relates generally to electrical connectors and moreparticularly to a flexible connector cable and method of fabricationthereof for providing high-density and reliable electricalinterconnections between conductive paths on substantially-planarsurfaces.

BACKGROUND OF THE INVENTION

Various applications exist in which it is desirable to form reliableelectrical connections to conductive paths on a substantially-planarsurface. For example, it is frequently desired to interconnect printedcircuit boards and/or flexible circuits to one another. Suchinterconnections have generally been formed utilizingsubstantially-conventional connector structures which are typicallybulky, expensive, and insufficiently reliable.

SUMMARY OF THE INVENTION

The present invention is directed to a flexible connector cable, andmethod of fabrication thereof, useful for interconnecting conductivepaths on substantially-planar surfaces.

In accordance with a preferred embodiment of the invention, a flatflexible laminar cable structure is provided comprised ofelectrically-insulative and electrically-conductive layers bonded to oneanother. The conductive layer is etched or otherwise formed to createdesired circuit paths. The insulative layer is etched or otherwiseformed to expose areas of the conductive layer where connections are tobe made.

In accordance with a significant aspect of the preferred embodiment,conductive contacts or bumps are formed on opposite surfaces of thecable wherever connections are to be made. The contacts are preferablyformed on a ductile material, e.g., gold, which exhibits plasticdeformation under applied pressure to form good electrical connectionsto a contiguous surface.

The novel features of the invention are set forth with particularity inthe appended claims. The invention will best be understood from thefollowing description when read in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective representative view of a flexible connectorcable in accordance with the present invention;

FIGS. 2 and 2A are enlarged sectional views, each partially broken away,illustrating a connector cable in accordance with the present invention,respectively connected to conductive paths on a single circuit board andon a pair of circuit boards;

FIG. 3 is a perspective representative view illustrating one applicationof a flexible connector cable in accordance with the present invention;

FIG. 4 is a perspective representative view illustrating anotherapplication of a connector cable in accordance with the presentinvention;

FIG. 5 illustrates a sequence of process steps in accordance with oneprocedure for fabricating an embodiment of the invention; and

FIG. 6 illustrates an alternative sequence of process steps forfabricating a connector cable in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Attention is now directed to FIGS. 1, 2, and 2A which illustrate aflexible connector cable 10 in accordance with the present invention forforming electrical connections to conductive paths on planar surfaces.Briefly, the connector cable 10 comprises a laminar structure formed bythe layer 12 of flexible electrically-insulative material and a layer 14formed of flexible electrically-conductive material.

As will be discussed in greater detail hereinafter, the layer 12 ofinsulative material comprises a strength member which supports theconductive layer 14 which is configured to define electrically-isolatedcircuit paths 16. In accordance with one significant aspect of theinvention, the circuit paths 16 have raised contacts 18 depositedthereon which are intended to contact conductive paths 20 on a planarcircuit board 22. In accordance with an important aspect of theinvention, as depicted in FIG. 2A, contacts 24 are also formed on thecable on the surface thereof opposite to the surface carrying thecontacts 18.

As can be seen in FIG. 2A, a void or opening 26 is formed in theinsulative layer 12 which exposes therethrough the conductive layer 14.The conductive layer 14 is preferably built up by plating and thecontact 24 is deposited on the builtup portion 28. Thus, with thecontacts 18 and 24 extending from opposite surfaces of the cable 10,connections can be made simultaneously to the conductive path 20 oncircuit board 22 and to the conductive path 30 on circuit board 32.Where through-connections are desired, the contacts 18 or 24 will bealigned and connected to the same circuit path 16. Wherethrough-connections are not desired, it is only necessary to provide acontact on one surface of the circuit path 16. The ability to providecontacts extending from opposite surfaces of the cable makes itexceedingly useful to provide high-density connections to conductivepaths on planar circuit boards as depicted in FIG. 2A. Although theinvention has maximum utility for such high-density applications, it isnevertheless also extremely useful for connecting to any single planarsurface as depicted in FIG. 2. It will, of course, be understood that asuitable clamping means (not shown) must be provided to press the cableagainst the circuit board, but a variety of such means can be used. Inits simplest form, the clamping means can merely comprise a rigid clampwhich bears against the cable and is bolted to the circuit board. Thecontacts 18 and 24 are preferably formed of a ductile material, such asgold, which exhibits plastic deformation when placed under a sufficientclamping force. For example, application of the clamping forcerepresented by the two force arrows in FIG. 2 causes the surface of thegold contacts 18 and 24 to minutely flow or deform, thus producing verygood and reliable electrical connections to the conductive paths 20 and30.

A flexible connector cable constructed in accordance with the presentinvention finds use in many applications for electrically connecting toconductive portions of planar surfaces. A somewhat unusual butsignificant application of the flexible cable 10 is illustrated in FIG.3 in which the cable is used to provide electrical connection toconductive areas within a stack 40 of rigid wafers of the type disclosedin such prior art patents as U.S. Pat. No. 3,705,332. The cited patentdiscloses an electrical circuit packaging structure in which a pluralityof electrically-conductive wafers are stacked together under pressureand in which X-Y axis conductors are formed within each wafer and Z-axisslugs are provided to interconnect wafers. When using such a waferstack, it is sometimes desirable to be able to bring out circuit pathsfrom within the stack to some external device. This is the arrangementrepresented in FIG. 3 in which the flexible cable 10 of FIG. 1 isrepresented as entering the stack 40. The end of the cable 10 remotefrom the stack 40 is shown as terminating in a conventional connector 44which can then be connected to some external device (not shown).

FIG. 4 illustrates a similar but somewhat different application of theflexible cable 10 for interconnecting two wafer stacks 46 and 48 of thetype described in the aforementioned U.S. Pat. No. 3,705,332.

Attention is now directed to FIG. 5 which illustrates a preferred methodof fabricating a flexible cable in accordance with the presentinvention. The method of FIG. 5 contemplates starting out with a coppersheet having a thickness on the order of 3 mils.

In step 1, the copper sheet is etched by well-known photographic andchemical etching procedures, to a thickness on the order of 1 milleaving mesas 56 wherever connections are to be made.

In step 2, a sheet 60 of electrically insulative material, havingopenings 62 to accommodate the mesas 56, is bonded to the copper sheet54. The electrically-insulative sheet 60 functions as a strength memberto support the circuit paths to be ultimately formed by the copper layer54. The sheet 60 preferably comprises a bonded plastic such as Polyimidehaving a thickness on the order of 1 mil. The voids or openings 62therein for accommodating the mesas 56 can be formed either by chemicaletching or by some mechanical means such as a punch. Still in step 2,the insulative layer 60 is preferably sanded to assure that the mesasurfaces are contiguous with the surface of the insulative sheet 60.

In step 3, gold contact buttons 66, 68 are plated both on the conductivelayer 54 and the exposed surfaces of mesas 56 wherever connections areultimately to be made.

In step 4, the desired circuit pattern is formed on the conductive layer54, preferably by photographic and chemical etching procedure. In thesimplest configuration, as is generally depicted in FIG. 1, theconductive layer 54 can be processed to merely form a plurality ofparallel electrically-isolated conductors. However, more complex circuitpatterns as are depicted in FIG. 5 can be defined for specialapplications. For example, FIG. 5 shows the formation of conductivepaths 70 and 72 which respectively extend from the plated contacts 74and 76, each path constituting an island electrically isolated from theremainder of the conductive layer 54. Note that each of the paths 70 and72 has been formed by removing portions of the conductive layer 54, asby chemical etching, along both sides of the paths leaving portions oflayer 54 to form the paths 70 and 72. The paths 70, 72 are essentiallyfully surrounded by the remaining portions of the conductive layer 54which remaining portions thus form a ground plane. The fact that aflexible connector cable in accordance with the present invention can befabricated to include a ground plane around the conductive paths 70, 72,makes it quite compatible with Planar Coax technology (as, for example,described in the aforecited U.S. Pat. No. 3,705,332) allowingimpedance-controlled stripline structures to be interconnected. Itshould, of course, be apparent that where ground plane systems are notrequired, all of the conductive layer 54 can be removed except for thoseportions which are left to define the desired circuit paths.

FIG. 5 illustrates circuit path 70 terminating in a mesa 80 having acontact 82, remote from the contact 74. Alternatively, FIG. 5illustrates conductive path 72 terminating in a ring of conductivematerial 84, electrically isolated from the ground plane material ofconductive layer 54. The ring of material 84 surrounds a hole 86 whichcan be utilized to receive a lead or pin, for example, which can then besoldered to the ring of material 84.

An alternative method of fabricating a flexible cable connector inaccordance with the invention is illustrated in FIG. 6. Step 1 depictsthe bonding together of a 1-mil sheet of copper 90 and a 1-mil sheet offlexible electrically-insulative material, such as Polyimide 92. Notethat the sheet 92 is shown with the openings or voids 94 formed therein.These openings 94 can be formed either by an appropriate mechanicalprocedure or chemical etching. In step 2 of FIG. 6 copper 96 is platedthrough the openings 94 of the insulative layer 92 to electricallyconnect to the conductive layer 90. Thereafter, in step 3, gold contacts98 are plated on the upper surface of the cable structure and goldcontacts 100 are plated on the lower surface of the cable structure. Asillustrated, the plated contacts can provide connections directlythrough the cable structure where desired.

In step 4 the conductive layer 90 is appropriately etched, as at 102 toform the desired circuit pattern.

From the foregoing, it should now be apparent that a flexible connectorcable has been disclosed herein suitable for providing high-densityconnections as well as high-reliability connections to conductive pathson planar structures. The cable can be inexpensively and preciselyfabricated by a sequence of well-known techniques such as bonding andchemical etching. Moreover, the construction of the cable is such thatit lends itself to fabrication by automatic or batch procedures. Forexample only, it should be apparent that the connector cable can beprocessed in long ribbon structures.

Although particular embodiments of the invention have been described andillustrated herein, it is recognized that modifications and variationsmay readily occur to those skilled in the art and consequently, it isintended that the claims be interpreted to cover such modifications andequivalents.

The embodiments of the invention in which an exclusive property orprivilege is claimed are described as follows:
 1. A method offabricating an interconnection cable including the steps of:forming alaminate structure comprised of flexible layers ofelectrically-insulative and electrically-conductive material; removingselective portions of said layer of electrically-conductive material todefine an electrically continuous ground plane and one or more circuitpaths each physically spaced and electrically-isolated from said groundplane and from each other circuit path; removing selected portions ofsaid layer of electrically-insulative material to expose the bottomsurface of said circuit paths; and depositing ductileelectrically-conductive material on the top surface of said circuitpaths and on the exposed bottom surface of said circuit paths.
 2. Amethod of fabricating an interconnection cable including the stepsof:forming a laminate structure comprised of flexible layers ofelectrically-insulative and electrically-conductive material; removingselective portions of said layer of electrically-conductive material todefine an electrically continuous ground plane and one or more circuitpaths each physically spaced and electrically-isolated from said groundplane and from each other circuit path; depositing a first raisedcontact on the top surface of each of said circuit paths; removingselected portions of said layer of electrically-insulative material forproviding access to the bottom surface of each of said circuit paths;and depositing a second raised contact on the bottom surface of each ofsaid circuit paths where said layer of electrically-insulative materialhas been removed.